A micro sample floating on a liquid surface aggregates, disperses, and aligns in different ways as determined by hydrophilicity and hydrophobicity. There is also a phenomenon in which individual particles align in specific directions as determined by physical conditions such as magnetic field, electric field, pressure, and temperature. Further, a technique is available that takes advantage that a micro sample on a liquid surface is not fixed, and freely controls the direction of the micro sample by controlling physical conditions such as magnetic field, electric field, pressure, and temperature. Observing such floating forms under an electron microscope is important in analyzing the physical properties of functional substances such as catalysts, drugs, and cosmetics, and fine crystals and organic powder materials, and the biology of small biological materials. It is, however, difficult with conventional techniques to make a high-magnification observation of a micro sample floating on a liquid surface. This is because the aqueous solution and the organic solvent evaporate in the vacuum environment of electron microscope observation.
As a countermeasure, a technique is proposed that makes an electron microscope observation after freezing a liquid below a freezing point, using a cool stage or a cryo stage. However, the technique is problematic, because the freezing of a liquid changes the shape of the micro sample, or the form of the micro sample, causing the micro sample to aggregate, disperse, align, or orientate differently. Another problem is that, despite the frozen sample, the water component evaporates in a vacuum. It, then, might be possible to use a technique that makes use of an oil as a liquid material that does not evaporate in a vacuum environment. However, a problem of such a technique is that the sample floating on the oil surface rapidly undergoes flowing movement under electron beam irradiation. The technique also has the charging problem. In another proposed method, a liquid is placed under atmospheric pressure, and a thin film is used to separate the liquid from the vacuum environment inside the tube of an electron microscope. However, the method still fails to enable observation of a micro sample floating on a liquid surface.
There is also a method in which a micro sample is sprinkled over a carbon paste or the like to mimic the floating of a micro sample on a liquid. This technique solves the charging problem, and does not involve flowability. The method thus advantageously makes the electron microscope observation easier. However, because the carbon paste quickly solidifies, the micro sample is quickly fixed before it fully aggregates, disperses, aligns, or orients. Thus, it cannot be said that the method successfully reproduces the form of a micro sample floating on a liquid surface. Further, because the micro sample is quickly fixed once being sprinkled over the carbon paste, it is not possible to control the direction of individual particles.
As a countermeasure, a technique is developed that uses an ionic liquid for the observation and control of a micro sample in a liquid under an electron microscope. For example, PTL 1 (WO2007/083756) solves the charging problem by applying an ionic liquid to a sample surface, and enables the actual shape of a sample to be observed with a scanning electron microscope and a transmission electron microscope.
PTL 2 (JP-A-2009-266741) enables observation of a sample floating on an ionic liquid after the sample is introduced into the ionic liquid held in a microgrid, a mesh, or the like.
PTL 3 (JP-A-2010-25656) applies an ionic liquid to a sample to prevent a sample surface from being exposed to the atmosphere.